CA2104320A1 - Control of nox reduction in flue gas flows - Google Patents
Control of nox reduction in flue gas flowsInfo
- Publication number
- CA2104320A1 CA2104320A1 CA002104320A CA2104320A CA2104320A1 CA 2104320 A1 CA2104320 A1 CA 2104320A1 CA 002104320 A CA002104320 A CA 002104320A CA 2104320 A CA2104320 A CA 2104320A CA 2104320 A1 CA2104320 A1 CA 2104320A1
- Authority
- CA
- Canada
- Prior art keywords
- flue gas
- nitrogeneous
- nox
- treatment agent
- treatment
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Abandoned
Links
Classifications
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01D—SEPARATION
- B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
- B01D53/34—Chemical or biological purification of waste gases
- B01D53/74—General processes for purification of waste gases; Apparatus or devices specially adapted therefor
- B01D53/86—Catalytic processes
- B01D53/8621—Removing nitrogen compounds
- B01D53/8625—Nitrogen oxides
- B01D53/8628—Processes characterised by a specific catalyst
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- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F23—COMBUSTION APPARATUS; COMBUSTION PROCESSES
- F23J—REMOVAL OR TREATMENT OF COMBUSTION PRODUCTS OR COMBUSTION RESIDUES; FLUES
- F23J15/00—Arrangements of devices for treating smoke or fumes
- F23J15/006—Layout of treatment plant
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01D—SEPARATION
- B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
- B01D53/30—Controlling by gas-analysis apparatus
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01D—SEPARATION
- B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
- B01D53/34—Chemical or biological purification of waste gases
- B01D53/346—Controlling the process
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01D—SEPARATION
- B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
- B01D53/34—Chemical or biological purification of waste gases
- B01D53/46—Removing components of defined structure
- B01D53/54—Nitrogen compounds
- B01D53/56—Nitrogen oxides
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01D—SEPARATION
- B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
- B01D53/34—Chemical or biological purification of waste gases
- B01D53/74—General processes for purification of waste gases; Apparatus or devices specially adapted therefor
- B01D53/86—Catalytic processes
- B01D53/8621—Removing nitrogen compounds
- B01D53/8625—Nitrogen oxides
-
- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F23—COMBUSTION APPARATUS; COMBUSTION PROCESSES
- F23J—REMOVAL OR TREATMENT OF COMBUSTION PRODUCTS OR COMBUSTION RESIDUES; FLUES
- F23J15/00—Arrangements of devices for treating smoke or fumes
- F23J15/02—Arrangements of devices for treating smoke or fumes of purifiers, e.g. for removing noxious material
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01D—SEPARATION
- B01D2251/00—Reactants
- B01D2251/20—Reductants
- B01D2251/206—Ammonium compounds
- B01D2251/2062—Ammonia
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01D—SEPARATION
- B01D2258/00—Sources of waste gases
- B01D2258/02—Other waste gases
- B01D2258/0283—Flue gases
-
- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F23—COMBUSTION APPARATUS; COMBUSTION PROCESSES
- F23J—REMOVAL OR TREATMENT OF COMBUSTION PRODUCTS OR COMBUSTION RESIDUES; FLUES
- F23J2217/00—Intercepting solids
- F23J2217/10—Intercepting solids by filters
- F23J2217/102—Intercepting solids by filters electrostatic
-
- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F23—COMBUSTION APPARATUS; COMBUSTION PROCESSES
- F23J—REMOVAL OR TREATMENT OF COMBUSTION PRODUCTS OR COMBUSTION RESIDUES; FLUES
- F23J2219/00—Treatment devices
- F23J2219/10—Catalytic reduction devices
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- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F23—COMBUSTION APPARATUS; COMBUSTION PROCESSES
- F23J—REMOVAL OR TREATMENT OF COMBUSTION PRODUCTS OR COMBUSTION RESIDUES; FLUES
- F23J2219/00—Treatment devices
- F23J2219/20—Non-catalytic reduction devices
Landscapes
- Engineering & Computer Science (AREA)
- Chemical & Material Sciences (AREA)
- Environmental & Geological Engineering (AREA)
- Analytical Chemistry (AREA)
- Oil, Petroleum & Natural Gas (AREA)
- General Chemical & Material Sciences (AREA)
- Chemical Kinetics & Catalysis (AREA)
- Biomedical Technology (AREA)
- Health & Medical Sciences (AREA)
- General Engineering & Computer Science (AREA)
- Mechanical Engineering (AREA)
- Treating Waste Gases (AREA)
- Exhaust Gas Treatment By Means Of Catalyst (AREA)
Abstract
CONTROL OF NOx REDUCTION IN FLUE GAS FLOWS
ABSTRACT OF THE DISCLOSURE
The NOx content in a flow of flue gas is reduced by passing the flue gas through a first treatment zone and a second treatment zone. A
nitrogeneous treatment agent is introduced into the first treatment zone for the selective non-catalytic reduction of part of the NOx, and the flue gas is thereafter passed through the second treatment zone which includes a catalyst for further selective catalytic reduction of the NOx. Optionally, a second nitrogeneous treatment agent is added to the flue gas in the second treatment zone. The quantity of NOx in the flue gas is detected intermediate the first and second treatment zones and, optionally, after the flue gas has left the second treatment zone. The quantity of ammonia in the flue gas exiting from the second treatment zone is also detected. The amounts of the treatment agents added to the flue gas are controlled responsive to the variations and absolute levels determined by these measurements.
ABSTRACT OF THE DISCLOSURE
The NOx content in a flow of flue gas is reduced by passing the flue gas through a first treatment zone and a second treatment zone. A
nitrogeneous treatment agent is introduced into the first treatment zone for the selective non-catalytic reduction of part of the NOx, and the flue gas is thereafter passed through the second treatment zone which includes a catalyst for further selective catalytic reduction of the NOx. Optionally, a second nitrogeneous treatment agent is added to the flue gas in the second treatment zone. The quantity of NOx in the flue gas is detected intermediate the first and second treatment zones and, optionally, after the flue gas has left the second treatment zone. The quantity of ammonia in the flue gas exiting from the second treatment zone is also detected. The amounts of the treatment agents added to the flue gas are controlled responsive to the variations and absolute levels determined by these measurements.
Description
210~320 CONTROL OF NOx REDUCTION IN FLUE GAS FLOWS
, BACKGROUND OF THE INVENTION
This lnventlon relstes to the reductlon of pollutants produced by bollers, and, more partlcularly, to the control of the process for reduclng NOx pollutants ln flue gas flows.
In a fossll-fuel power plant, coal, gas, or oll 1~ burned to boll water to form steam., The steam drlves a turblne and thence an electrlc generator, produclng electrlclty. Besldes heat, the combustlon produces gaseous pollutants such as sulfur and nltrogen oxldes, and a solld partlculate termed fly ash. Envlronmental protectlon laws mandate that the amounts of the gaseous and solld pollutants be maintalned at acceptably low levels.
The present lnventlon deals wlth reduclng and malntalnlng the smog-produclng nltrogen oxldes, known generally as NOx,,wlthln acceptable levels.
It 18 known that the NOY level ln flue gas ls lowered by reactlng the NOx wlth ammonla, to produce harmless nltrogen and water as reactlon products.
The reactlon can occur at relatlvely hl~h temperatures wlthout a catalyst, or at lower temperatures ln the presence of a catal~st. The former 18 known ln the art &S select'lve non-catalytlc reductlon (SNCR), and the latter ls known as selectlve catalytlc reductlon (SCR). In a process modiflcatlon, both SNCR and SCR may be performed slmultaneously on the same flue gas stream. The SNSR ls accompllshed ln a flrst zone shortly after the hot flue ~as leaves the furnace, and the SCR ln a second zone through whlch the cooler flue gas subsequently passes.
Ammonia must be present for both SNCR and SCR
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, ' . : . , 21~320 reactlons to occur. Slnce provldlng a sufflclent amount of gaseous or llquld ammonla for both reactions ls expenslve, the required ammonla molecules may be provlded bg lntroduclng a nitrogenous compound such as urea lnto the hot flue gas ln the flrst treatment zone. The urea decomposes at hlgh temperatures, provldlng ammonla for the SNCR reactlon. The urea ls usuall~ added ln excess of that requlred for the SNCR reaotlon, and the SCR helps to remove the resultlng excess ammonla from the flue gas ~tream. Addltlonal nltrogenous compounds or ammonla ltself can be added to the flue gas at the second treatment zone, lf requlred to complete the SCR reactlon.
Although SNCR and SCR are generally effectlve ln reduclng NOX content of the flue gas, the addltlon of ammonla leads to another potentlal source of pollutlon. If the ammonla lntroduced lnto the ~lue gas 18 not entlrel~ consumed ln the reactlons wlth NOY, some ammonla remalns ln the flue gas and passes to the atmosphere, a consequence termed "ammonla sllp". Ammonla sllp ls often observed ln the eYhaust plumes of those power plants that use SNCR or SCR to reduce the NOX aontent of the flue gas.
There 18 a need for some approach to achlevlng reductlon of NOX ln flue gas to acceptablg low llmlts, and also malntalnlng ammonla sllp below the regulatorg llmlts. The present lnventlon fulfllls thls need, and further provldes related advantages.
SUMMARY OF THE INVENTION
The present lnventlon provldes a method for reduclng the NOx content ln a flow of flue gas, and .
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2lo432o slmultaneously malntalnlng the ammonla sllp wlthln acceptable llmlts. The present approach provldes a control method that operates ln an lteratlve manner to achleve lts obJectlves, and can therefore adJust to changes ln operatlng characterlstlcs of the boller. Mlnlmal modlflcatlon to the power plant ls requlred ln order to lmplement the approach.
In accordance wlth the lnventlon, a method of reduclng NOx from a flow of flue gas produced by a burner comprlses the steps of passlng the flue gas through a flrst treatment zone, and, substantlally slmultaneously wlth sald passlng, selectlvely lntroduclng a nltrogeneous treatment agent lnto the flrst treatment zone for the selectlve non-catalytlc reductlon of a flrst portlon of the NOx from the flue gas passlng through the flrst treatment zone.
The nltrogeneous treatment agent 18 lntroduced ln an excesslve amount above that requlred for the non-catalytlc reductlon of the flrst portlon. The method further lnvolves passlng the flue gas, contalnlng such excesslve amount of treatment agent, through a second treatment zone whlch lncludes a catalyst thereln whereln a second portlon of the NO~
wlthln the flue gas 18 removed as a result of a reactlon of the NOx wlth such excesslve treatment agent ln the presence of the catalyst. Optlonally, a second nltrogeneous treatment agent can be lntroduced lnto the flue gas ln the second treatment zone. The quantlty of NOx ln the flue gas ls detected lntermedlate the flrst and second treatment zones, and, optlonally, after the flue gas leaves the second treatment zone. The quantlty of ammonia ls detected ln the flue gas exltlng from the second treatment zone.
The quantltles of c the addltlon of the treatment agent and ~econd treatment agent are varled accordlng to the results of the NOx and "
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.. 2104320 ammonla meaæurements. The quantlty of treatment agent lntroduced lnto the flrst treatment zone i8 lncreased so long as the followlng crlterla are met: First, that the quantity of NOx measured between the flrst and second treatment zones, over a perlod of tlme, ls generally decreaslng wlth an lncreaslng addltlon of the treatment agent, and, second, that the quantlt~ of ammonla detected ln the flue gas leavlng the second treatment zone, over a perlod of tlme, 18 generally below a predetermlned maxlmum llmlt. The second treatment agent ls lntroduced lnto the flue gas 80 long as the followlng crlterla are met: Flrst, that the quantlt~ of NOx measured between the flrst and second treatment zones, over a perlod of tlme, ls not generally decreaslng as a result of a further addltlon of the treatment agent, and, second, that the quantlt~ of ammonla detected ln the flue gas after lt leaves the second reactlon zone (the ammonia sllp), over a perlod of tlme, ls generally below a predetermined maxlmum llmlt. The addltlon of the second treatment agent can be lncreased as long as the quantlt~ of NOx measured after the flue gas leaves the ~econd treatment zone has not substantlally reached a predetermlned target control polnt.
Thls approach 18 preferabl~ lmplemented ln an lncremental, lteratlve control procedure that ad~usts the additlon rates of the treatment agents responslve to the measurements of the NOx and ammonla ln the flue gas. It can be spplled to varlous types of power plants, both as an lnltlal lnstallatlon and as a retroflt to bollers to lmprove their operational characterlstlcs. Other features and advantages of the present lnventlon wlll be apparent from the followlng more detalled descrlptlon of the preferred embodlment, taken ln . ~ :. : . : , . .
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conJunction wlth the accompanylng drawlngs, whlch illustrate, b~ way of example, the prlnclple~ of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
Flgure 1 ls a schematlc drawlng of a power plant;
Flgure 2 ls a graph of NOx and ammonla concentratlons after the completlon of SNCR as a function of added (treatment agent) urea concentratlon, for reactlon temperatures less than 2000F;
Flgure ~ 1~ 8 graph of NOx and ammonla concentratlons after the completlon of SNCR as a functlon of added (treatment agent) urea concentratlon, for reactlon temperatures of more than 2000F;
Flgure 4 18 a graph of NOs and ammonla concentratlons after the completlon of SCR as a functlon of ammonla concentratlon at the inlet of the SCR process;
Flgure 5 18 a process flow dlagram for the control approach of the inventlon; and Flgure 6 ls a process flow diagram for the iteratlve control procedure.
DETAILED DESCRIPTION OF THE INVENTION
Flgure 1 deplcts a power plant 20 ln whlch fossll fuel such as coal, gas, or oll 18 combusted wlth alr. Alr ls drawn .lnto an lntake 22 by a blower 24, whlch forces the alr along a combustlon air condult 26 and through an alr side 28 of a L'Jungstrom t~pe rotary heat exchanger ~0. The alr , , -6- 2~0432~
introduced through the condult 26 ls heated durlng its passage through the heat exchanger 30, and flows to a furnace 32. Fuel ls introduced lnto the furnace 32 from a fuel source 34. The fuel burns ln the lntake alr, producing combustlon products known as flue gas. The flue gas contalns a variety of constltuents, lncludlng potentlal pollutants such as sulfur oxldes (SOx), nltrogen oxides (NOx), and partlculate matter resultlng from combustlon. The flue gas passes lnto a boller 36 and through a steam generator 38. In the steam generator, water ls vaporlzed to steam, whlch flows to a turbine 40.
The turblne 40 ls connected to an electrlcal generator that produces electrlcal power, the deslrable product of the power plant 20.
The flue gas flows through a hot gas condult 42 and lnto a flue gas slde 44 of the heat exchanger 30. Eeat transfer elements ln the heat exchanger 30 are heated by the hot gas, and thereafter are rotated to the alr slde to transfer thelr heat to the combustlon alr ln the combustlon alr condult 26. After passlng through the heat exchanger 30, the cooler flue gas enters a partlculate collector 46, whlch typlcally ls an electrostatlc preclpltator. The flue gas, from whlch most of the partlculate 18 removed, ls forced by a blower 48 up an exhaust stack 50 and to the atmosphere.
The precedlng dlscusslon has provlded a general descrlptlon of a power plant. Many aspects not relevant to the present inventlon have been omltted from the dlscusslon. The present lnventlon may also be used wlth other conflguratlons of power plant, and Flgure 1 deplcts only the preferred setting for practlce of the lniventlon.
A nltrogenous treatm~nt agent, preferably urea, is lnJected lnto the hot flue gas ln the boiler 36 from a treatment agent source 51. In the ' .
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': , ' ' ' ' - 2~432o hot flue gas, the urea decomposes to produce ammonla. The ammonla ln turn react~ wlth NOx ln the flue gas by selectlve non-catalytlc reductlon (SNCR). Thls reaction consumes NOx and ammonla and produces nltrogen gas and water. The SNCR reactlon occurs over a wlde temperature range, but is preferably conducted at a temperature of from about 1600F to about 2000F as the flue Bas flows along the hot gas condult 42.
SNCR can reduce the NOx content only as far as permltted by the equlllbrium condltlons and the klnetics of the reactlon. The ablllty to lntroduce large amounts of ammonia lnto the flue gas stream to increase the consumption of NOs by SNCR is also llmlted bg ammonla sllp, whlch 18 the passage of unreacted ammonia to the stack gas and thence to the environment. Ammonia is a toxic regulated substance, and lt cannot be added ln ever-lncreaslng amounts to the flue gas for the purpose of SNCR, because a fraction of the added ammonla reaches the envlronment as a pollutant.
Wlthln the ammonia emlsslon llmlt~ lmposed by ammonla sllp, the NOx content of the flue gas i8 hlgher than deslred after the SNCR reactlon occurs.
Addltlonal NOx can therefore be removed by ~electlve catalytlc reductlon (SCR). To accomplish SCR, the mlxture of ammonla and flue gas ls passed over a catalyst, whlch catalyzes the reactlon between NOx and ammonia further toward completion at lower temperatures than for the SNCR. The SCR i~
preferably conducted at a temperature of from about 300F to about 950F in the presence of a material operable to catalyze the reactlon of NOx and ammonla, preferably vanadla, tungsten, zeollte, noble metals, or transltion metals. The catalyst ls supported ln a fixed catalyst bed 52. In order to gain more surface area for the catalyst, lt may also :
be applled to hot-end heat transfer elements 54 of the rotary heat exchanger 30. For SNCR the urea ls usually required ln quantitle~ in an excess to the stoichiometric ratlo In relation to ammonla, so that some unreacted ammonia remains for the SCR
reaction. A second treatment agent, preferably ammonla ln gsseous form, may be lnJected from a source 56 into the flowlng flue gas Just upstream of the catalyst bed 52. The SCR results ln the reactlon of the ammonia and NOx in the flue gas 80 as to reduce both to acceptabl~ low levels.
As ls now apparent, the concentratlons of NOx and ammonla lost from the SNCR are lnterrelated to the urea addltlon. As shown ln Flgure 2, ln the temperature range of less than 2000F the NOs content decrease~ monotonlcally wlth lncreaslng urea addltlon, whlle the ammonla 811p lncreases wlth increaslng urea addltlon. As shown ln Flgure ~, at hlgher temperatures the NOx reaches a mlnlmum value and then lncreases, wlth lncreaslng urea addltlons.
Slmllarly, the concentratlons of NOx and ammonla sllp from the SCR are related to the ammonla concentratlon at the lnlet of the SCR process. As shown ln Flgure 4, the NOx content decreases monotonlcally wlth lncreaslng ammonla concentratlon at the lnlet of the SCR process, whlle the ammonla sllp lncreases. The observations of Flgures 2-4 are useful ln deslgnlng the control strategy for'the comblned use of SCR and SNCR.
The portlon of the boller 36 and hot gas condult 42 havlng the proper temperature for SNCR, and in whlch the treatment agent ls lntroduced, ls termed a flrst treatment zone 58. The portlon of the hot gas path havlng the catalyst ln the flue gas stream, and ln which the second treatment agent (lf any) ls introduced, is termed a second treatment zone 60. The second treatment zone 60 may be .
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_9_ consldered as havlng two subzones, one havlng the flxed bed catalyst 52 and the other havlng the catalyst supported on the heat transfer elements of the heat exchanger 30.
Accordlng to the present lnvention, det~ctors are provlded to monltor the flue gas composltlon at æeveral locatlons. A flrst NOx detector 62 18 posltloned ln the hot gas condult 42 upstream of the second treatment zone 60, and preferabl~ between the flrst treatment zone 58 and the second treatment zone 60. The detector 60 senses the concentratlon and thence quantlty of NOx ln the flue Bas ~tream at this polnt. An ammonla detector 64 ls posltloned ln the hot gas condult 42 downstream of the second treatment zone 60. Flgure 1 shows the detector 64 lmmedlatel~ downstream of the zone 60, but lt could be farther downstream a~ far as the exhaust stack 50. The detector 64 senses the concentratlon and thence the quantlty of ammonla ln the flue gas stream after lt has left the second treatment zone 60. Thls flgure 18 termed the ~ammonla sllp".
Optlonall~, a second NO~ detector 66 18 posltloned downstream of the second treatment zone 60 ln the hot gas condult 42 or the stack 50. The detector 66 senses the concentratlon and thence the quantlty of NOx ln the flue gas stream after lt has left the second treatment zone 60.
Instrumentatlon for use as the detectors 62, 64, and 66 18 avallable commerclall~. The NO~
detectors 62 and 66, as well as the ammonla detector 64, are preferably the Model 6000 emlsslon monitoring sy~tem made b~ Alr Instrument Measurements, Inc.
Control of the NOx reductlon ln the flue gas ls achieved by uslng the results measured by the detectors 62, 64, and 66 to control the flows of treatment agents from the sources 51 and 56.
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Addltlonally, lnformation on the amount of fuel consumed, termed the boller load, and the combustlon temperature, -are provlded. The analysls of the results of the detectors and the control of the sources ls accompllshed by a controller 68. The controller 68 preferabl~ lncludes a mlcrocomputer programmed to follow the control procedures dlscussed next.
Flgure 5 deplcts the process for removlng NOx from the flow of flue gas. The flue gas 18 passed through the flrst treatment zone 58, numeral 70.
The treatment agent, preferably ures, 18 lntroduced into the flrst treatment zone 58 to ml~ with the flue gas, numeral 72. The NOx ln the flue gas then undergoes SNCR by reactlon wlth the treatment agent. The flue gas, wlth a reduced NOx content, passes through the second treatment zone 60 to undergo SCR, numeral 74. Optlonally, a second treatment agent, preferably ammonla, 18 a~ded at the upstream end of the second treatment zone 60, numeral 74. The NOx content of the flue gas 18 detected by detector 62 between the flrst trestment zone 58 and the second treatment zone 60, numeral 76. The ammonla sllp of the flue gas ls detected by detector 64 after the flue gas leaves the second treatment zone 60, numeral 78. In thls preferred embodlment, the NOx content of the flue gas 18 detected by detector 66 after the flue gas leaves the second treatment zone 60, numeral 80.
The measured values of the detectors 62, 64, and 66 are provlded to the controller 68, and the changes ln the flow rates of the treatment agents are determlned, numeral 80. The detalls of the procedure are descrlbed subsiequently ln relatlon to Flgures 2-4, but ln genera~, the procedures are as follows. The quantlty of urea added to the flrst treatment zone 58 ls lncreased, so long as the . .
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~i~4320 followlng crlterla are met: Flrst, the quantlty of NO~ measured by detector 62, over a perlod of tlme, is generally decreaslng wlth lncreaslng addltlons of the treatment a8ent (l.e., a negatlve slope of the NOx-urea curve of Flgure 2 or Flgure ~), and, second, the quantlty of ammonla measured at the detector 64, over a perlod of tlme, ls generally below a predetermlned maxlmum llmlt. On the other hand, lf the quantlty of NOx measured b~ detector 62, over a perlod of tlme, 18 generally lncreaslng wlth lncreaslng addltlons of the flrst treatment agent (l.e., a posltlve slope ln the NOx-urea curve of Flgure 3), the urea additlon rate 18 reduced.
Ammonla 18 added to the flue gas from source 56, 80 long as the followlng crlterla are belng met: Flrst, the quantlty of NOx measured at detector 62, over a perlod of tlme, 18 not generall~
decreaslng wlth lncreaslng addltlons of the treatment agent, and, second, the quantlty of ammonia measured at detector 64, over a perlod of tlme, 18 generally below a predetermlned maxlmum llmlt. The addltlon of the second treatment agent 18 lncreased as long as the quantlty of NOx measured at detector 66, over a perlod of tlme, ha~ not substantlall~ reached (l.e., 18 greater than) a predetermlned target control polnt. After these determlnatlons have been made, the flrst treatment agent flow rate 18 responslvel~ varled by controlllng the source of the treatment agent 51, numeral 84, and the ammonla flow rate (lf any) 18 responslvely varled bg controlllng the ammonla source 56, numeral 86.
Flgure 6 deplcts one procedure that may be used b~ the controller 68 ln performlng the determlnatlons lndlcated at -numeral 82, and the relatlon of these determlnatlons to the determlnatlons of ga~ content 62, 64, and 66, and .
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the varglng of the treatment agent, here urea 84, and ammonla 86. In general, thls procedure provldes that the treatment agent 1s added to the maxlmum extent posslble, and changes ln the addltlon of the treatment agent are the prlmar~ controlled quantlty. The sgstem performance 18 then trlmmed with addltlons of the second treatment agent, lf needed and permltted bg the clrcumstances.
In thls proce~s, the boller load 1~ checked, numeral 100, and the combustlon temperature ls checked, numeral 102. If elther 18 not stable, l.e., lf elther has changed slnce the last check, a FLAG ls set to 1, numeral 104, to lndlcate that the selectlve noncatalgtlc reductlon process must flrst be optlmlzed. Otherwlse, the FLAG remalns as lt was. In each case, readlngs are then taken from the detector 62 and the detector 64. The ammonla sllp of the stack gas as measured by detector 64 ls compared wlth a preestabllshed regulatory llmlt, numeral 106. If the ammonla sllp 18 below the llmlt, the stack NOY content 18 measured, numeral 66. If the ammonla sllp 18 not below the llmlt, a varlable STEP whlch lndlcatea the next change ln a flow rate 18 set to a negatlve value, numeral 108.
If the path through the NOX measurement, numeral 66 18 followed, the stack NOx content 18 compared wlth a preestabllshed goal, numeral 110. If the stack NOx content as measured bg detector 66 18 less than the goal, the process proceeds to the settlng of STEP to a negatlve value, numeral 108.
All paths pass through a check of the FLAG, numeral 114. If FLAG ls 1, a preestabllshed waltlng tlme after a change ln the flow rate of the treatment agent ls checked, numeral 116, and the change ln the NOx content at detector 62 after a wait time is checked, numeral 118. The slope of the curve of NOx as a functlon of flow rate of the flrst ;:
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treatment agent 18 thereby evaluated. If the slope is not between two preselected control polnts, the direction of the variation in the urea flow rate 51 (the prevlously established value of STEP) is selected, numeral 122, and the urea flow rate is varied, numeral 84. In the case where the slope ls less than the lower preselected control polnt, STEP
is set positive. If the slope 1~ hlgher than the upper preselected control polnt, STEP ls set negatlve. If the slope is between the two preselected control points, FLAG 18 set equsl to zero, numeral 124, and a change ln the ammonla flow rate 56 ls lnltlated, numeral 126. After waltlng a preselected amount of tlme, numeral 128, the ammonla flow rate change 18 lmplemented, numeral 86.
If FLAG 18 found to be zero at numeral 114, the process sklps dlrectly to numeral 128.
Thls entlre control procedure then repeats contlnuously, controlllng the urea and ammonla lnJectlon rates 80 that the N0~ snd ammonla are wlthln establlshed llmlts.
The approach of the lnventlon provldes a process for contiolllng the reductlon of NOx ln flue gas, whlle avoldlng ammonla sllp above establlshed llmlts. It 18 usable on a varlety of power plants and other combustlon devlces. Although a partlcular embodlment of the lnventlon has been descrlbed ln detall for purposes of lllustratlon, varlous modlflcatlons ma~ be made wlthout dapartlng from the splrlt and scope of the lnventlon. Accordlngl~, the lnventlon 18 not to be llmlted except as by the appended clslms.
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, BACKGROUND OF THE INVENTION
This lnventlon relstes to the reductlon of pollutants produced by bollers, and, more partlcularly, to the control of the process for reduclng NOx pollutants ln flue gas flows.
In a fossll-fuel power plant, coal, gas, or oll 1~ burned to boll water to form steam., The steam drlves a turblne and thence an electrlc generator, produclng electrlclty. Besldes heat, the combustlon produces gaseous pollutants such as sulfur and nltrogen oxldes, and a solld partlculate termed fly ash. Envlronmental protectlon laws mandate that the amounts of the gaseous and solld pollutants be maintalned at acceptably low levels.
The present lnventlon deals wlth reduclng and malntalnlng the smog-produclng nltrogen oxldes, known generally as NOx,,wlthln acceptable levels.
It 18 known that the NOY level ln flue gas ls lowered by reactlng the NOx wlth ammonla, to produce harmless nltrogen and water as reactlon products.
The reactlon can occur at relatlvely hl~h temperatures wlthout a catalyst, or at lower temperatures ln the presence of a catal~st. The former 18 known ln the art &S select'lve non-catalytlc reductlon (SNCR), and the latter ls known as selectlve catalytlc reductlon (SCR). In a process modiflcatlon, both SNCR and SCR may be performed slmultaneously on the same flue gas stream. The SNSR ls accompllshed ln a flrst zone shortly after the hot flue ~as leaves the furnace, and the SCR ln a second zone through whlch the cooler flue gas subsequently passes.
Ammonia must be present for both SNCR and SCR
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, ' . : . , 21~320 reactlons to occur. Slnce provldlng a sufflclent amount of gaseous or llquld ammonla for both reactions ls expenslve, the required ammonla molecules may be provlded bg lntroduclng a nitrogenous compound such as urea lnto the hot flue gas ln the flrst treatment zone. The urea decomposes at hlgh temperatures, provldlng ammonla for the SNCR reactlon. The urea ls usuall~ added ln excess of that requlred for the SNCR reaotlon, and the SCR helps to remove the resultlng excess ammonla from the flue gas ~tream. Addltlonal nltrogenous compounds or ammonla ltself can be added to the flue gas at the second treatment zone, lf requlred to complete the SCR reactlon.
Although SNCR and SCR are generally effectlve ln reduclng NOX content of the flue gas, the addltlon of ammonla leads to another potentlal source of pollutlon. If the ammonla lntroduced lnto the ~lue gas 18 not entlrel~ consumed ln the reactlons wlth NOY, some ammonla remalns ln the flue gas and passes to the atmosphere, a consequence termed "ammonla sllp". Ammonla sllp ls often observed ln the eYhaust plumes of those power plants that use SNCR or SCR to reduce the NOX aontent of the flue gas.
There 18 a need for some approach to achlevlng reductlon of NOX ln flue gas to acceptablg low llmlts, and also malntalnlng ammonla sllp below the regulatorg llmlts. The present lnventlon fulfllls thls need, and further provldes related advantages.
SUMMARY OF THE INVENTION
The present lnventlon provldes a method for reduclng the NOx content ln a flow of flue gas, and .
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2lo432o slmultaneously malntalnlng the ammonla sllp wlthln acceptable llmlts. The present approach provldes a control method that operates ln an lteratlve manner to achleve lts obJectlves, and can therefore adJust to changes ln operatlng characterlstlcs of the boller. Mlnlmal modlflcatlon to the power plant ls requlred ln order to lmplement the approach.
In accordance wlth the lnventlon, a method of reduclng NOx from a flow of flue gas produced by a burner comprlses the steps of passlng the flue gas through a flrst treatment zone, and, substantlally slmultaneously wlth sald passlng, selectlvely lntroduclng a nltrogeneous treatment agent lnto the flrst treatment zone for the selectlve non-catalytlc reductlon of a flrst portlon of the NOx from the flue gas passlng through the flrst treatment zone.
The nltrogeneous treatment agent 18 lntroduced ln an excesslve amount above that requlred for the non-catalytlc reductlon of the flrst portlon. The method further lnvolves passlng the flue gas, contalnlng such excesslve amount of treatment agent, through a second treatment zone whlch lncludes a catalyst thereln whereln a second portlon of the NO~
wlthln the flue gas 18 removed as a result of a reactlon of the NOx wlth such excesslve treatment agent ln the presence of the catalyst. Optlonally, a second nltrogeneous treatment agent can be lntroduced lnto the flue gas ln the second treatment zone. The quantlty of NOx ln the flue gas ls detected lntermedlate the flrst and second treatment zones, and, optlonally, after the flue gas leaves the second treatment zone. The quantlty of ammonia ls detected ln the flue gas exltlng from the second treatment zone.
The quantltles of c the addltlon of the treatment agent and ~econd treatment agent are varled accordlng to the results of the NOx and "
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.. 2104320 ammonla meaæurements. The quantlty of treatment agent lntroduced lnto the flrst treatment zone i8 lncreased so long as the followlng crlterla are met: First, that the quantity of NOx measured between the flrst and second treatment zones, over a perlod of tlme, ls generally decreaslng wlth an lncreaslng addltlon of the treatment agent, and, second, that the quantlt~ of ammonla detected ln the flue gas leavlng the second treatment zone, over a perlod of tlme, 18 generally below a predetermlned maxlmum llmlt. The second treatment agent ls lntroduced lnto the flue gas 80 long as the followlng crlterla are met: Flrst, that the quantlt~ of NOx measured between the flrst and second treatment zones, over a perlod of tlme, ls not generally decreaslng as a result of a further addltlon of the treatment agent, and, second, that the quantlt~ of ammonla detected ln the flue gas after lt leaves the second reactlon zone (the ammonia sllp), over a perlod of tlme, ls generally below a predetermined maxlmum llmlt. The addltlon of the second treatment agent can be lncreased as long as the quantlt~ of NOx measured after the flue gas leaves the ~econd treatment zone has not substantlally reached a predetermlned target control polnt.
Thls approach 18 preferabl~ lmplemented ln an lncremental, lteratlve control procedure that ad~usts the additlon rates of the treatment agents responslve to the measurements of the NOx and ammonla ln the flue gas. It can be spplled to varlous types of power plants, both as an lnltlal lnstallatlon and as a retroflt to bollers to lmprove their operational characterlstlcs. Other features and advantages of the present lnventlon wlll be apparent from the followlng more detalled descrlptlon of the preferred embodlment, taken ln . ~ :. : . : , . .
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conJunction wlth the accompanylng drawlngs, whlch illustrate, b~ way of example, the prlnclple~ of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
Flgure 1 ls a schematlc drawlng of a power plant;
Flgure 2 ls a graph of NOx and ammonla concentratlons after the completlon of SNCR as a function of added (treatment agent) urea concentratlon, for reactlon temperatures less than 2000F;
Flgure ~ 1~ 8 graph of NOx and ammonla concentratlons after the completlon of SNCR as a functlon of added (treatment agent) urea concentratlon, for reactlon temperatures of more than 2000F;
Flgure 4 18 a graph of NOs and ammonla concentratlons after the completlon of SCR as a functlon of ammonla concentratlon at the inlet of the SCR process;
Flgure 5 18 a process flow dlagram for the control approach of the inventlon; and Flgure 6 ls a process flow diagram for the iteratlve control procedure.
DETAILED DESCRIPTION OF THE INVENTION
Flgure 1 deplcts a power plant 20 ln whlch fossll fuel such as coal, gas, or oll 18 combusted wlth alr. Alr ls drawn .lnto an lntake 22 by a blower 24, whlch forces the alr along a combustlon air condult 26 and through an alr side 28 of a L'Jungstrom t~pe rotary heat exchanger ~0. The alr , , -6- 2~0432~
introduced through the condult 26 ls heated durlng its passage through the heat exchanger 30, and flows to a furnace 32. Fuel ls introduced lnto the furnace 32 from a fuel source 34. The fuel burns ln the lntake alr, producing combustlon products known as flue gas. The flue gas contalns a variety of constltuents, lncludlng potentlal pollutants such as sulfur oxldes (SOx), nltrogen oxides (NOx), and partlculate matter resultlng from combustlon. The flue gas passes lnto a boller 36 and through a steam generator 38. In the steam generator, water ls vaporlzed to steam, whlch flows to a turbine 40.
The turblne 40 ls connected to an electrlcal generator that produces electrlcal power, the deslrable product of the power plant 20.
The flue gas flows through a hot gas condult 42 and lnto a flue gas slde 44 of the heat exchanger 30. Eeat transfer elements ln the heat exchanger 30 are heated by the hot gas, and thereafter are rotated to the alr slde to transfer thelr heat to the combustlon alr ln the combustlon alr condult 26. After passlng through the heat exchanger 30, the cooler flue gas enters a partlculate collector 46, whlch typlcally ls an electrostatlc preclpltator. The flue gas, from whlch most of the partlculate 18 removed, ls forced by a blower 48 up an exhaust stack 50 and to the atmosphere.
The precedlng dlscusslon has provlded a general descrlptlon of a power plant. Many aspects not relevant to the present inventlon have been omltted from the dlscusslon. The present lnventlon may also be used wlth other conflguratlons of power plant, and Flgure 1 deplcts only the preferred setting for practlce of the lniventlon.
A nltrogenous treatm~nt agent, preferably urea, is lnJected lnto the hot flue gas ln the boiler 36 from a treatment agent source 51. In the ' .
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': , ' ' ' ' - 2~432o hot flue gas, the urea decomposes to produce ammonla. The ammonla ln turn react~ wlth NOx ln the flue gas by selectlve non-catalytlc reductlon (SNCR). Thls reaction consumes NOx and ammonla and produces nltrogen gas and water. The SNCR reactlon occurs over a wlde temperature range, but is preferably conducted at a temperature of from about 1600F to about 2000F as the flue Bas flows along the hot gas condult 42.
SNCR can reduce the NOx content only as far as permltted by the equlllbrium condltlons and the klnetics of the reactlon. The ablllty to lntroduce large amounts of ammonia lnto the flue gas stream to increase the consumption of NOs by SNCR is also llmlted bg ammonla sllp, whlch 18 the passage of unreacted ammonia to the stack gas and thence to the environment. Ammonia is a toxic regulated substance, and lt cannot be added ln ever-lncreaslng amounts to the flue gas for the purpose of SNCR, because a fraction of the added ammonla reaches the envlronment as a pollutant.
Wlthln the ammonia emlsslon llmlt~ lmposed by ammonla sllp, the NOx content of the flue gas i8 hlgher than deslred after the SNCR reactlon occurs.
Addltlonal NOx can therefore be removed by ~electlve catalytlc reductlon (SCR). To accomplish SCR, the mlxture of ammonla and flue gas ls passed over a catalyst, whlch catalyzes the reactlon between NOx and ammonia further toward completion at lower temperatures than for the SNCR. The SCR i~
preferably conducted at a temperature of from about 300F to about 950F in the presence of a material operable to catalyze the reactlon of NOx and ammonla, preferably vanadla, tungsten, zeollte, noble metals, or transltion metals. The catalyst ls supported ln a fixed catalyst bed 52. In order to gain more surface area for the catalyst, lt may also :
be applled to hot-end heat transfer elements 54 of the rotary heat exchanger 30. For SNCR the urea ls usually required ln quantitle~ in an excess to the stoichiometric ratlo In relation to ammonla, so that some unreacted ammonia remains for the SCR
reaction. A second treatment agent, preferably ammonla ln gsseous form, may be lnJected from a source 56 into the flowlng flue gas Just upstream of the catalyst bed 52. The SCR results ln the reactlon of the ammonia and NOx in the flue gas 80 as to reduce both to acceptabl~ low levels.
As ls now apparent, the concentratlons of NOx and ammonla lost from the SNCR are lnterrelated to the urea addltlon. As shown ln Flgure 2, ln the temperature range of less than 2000F the NOs content decrease~ monotonlcally wlth lncreaslng urea addltlon, whlle the ammonla 811p lncreases wlth increaslng urea addltlon. As shown ln Flgure ~, at hlgher temperatures the NOx reaches a mlnlmum value and then lncreases, wlth lncreaslng urea addltlons.
Slmllarly, the concentratlons of NOx and ammonla sllp from the SCR are related to the ammonla concentratlon at the lnlet of the SCR process. As shown ln Flgure 4, the NOx content decreases monotonlcally wlth lncreaslng ammonla concentratlon at the lnlet of the SCR process, whlle the ammonla sllp lncreases. The observations of Flgures 2-4 are useful ln deslgnlng the control strategy for'the comblned use of SCR and SNCR.
The portlon of the boller 36 and hot gas condult 42 havlng the proper temperature for SNCR, and in whlch the treatment agent ls lntroduced, ls termed a flrst treatment zone 58. The portlon of the hot gas path havlng the catalyst ln the flue gas stream, and ln which the second treatment agent (lf any) ls introduced, is termed a second treatment zone 60. The second treatment zone 60 may be .
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_9_ consldered as havlng two subzones, one havlng the flxed bed catalyst 52 and the other havlng the catalyst supported on the heat transfer elements of the heat exchanger 30.
Accordlng to the present lnvention, det~ctors are provlded to monltor the flue gas composltlon at æeveral locatlons. A flrst NOx detector 62 18 posltloned ln the hot gas condult 42 upstream of the second treatment zone 60, and preferabl~ between the flrst treatment zone 58 and the second treatment zone 60. The detector 60 senses the concentratlon and thence quantlty of NOx ln the flue Bas ~tream at this polnt. An ammonla detector 64 ls posltloned ln the hot gas condult 42 downstream of the second treatment zone 60. Flgure 1 shows the detector 64 lmmedlatel~ downstream of the zone 60, but lt could be farther downstream a~ far as the exhaust stack 50. The detector 64 senses the concentratlon and thence the quantlty of ammonla ln the flue gas stream after lt has left the second treatment zone 60. Thls flgure 18 termed the ~ammonla sllp".
Optlonall~, a second NO~ detector 66 18 posltloned downstream of the second treatment zone 60 ln the hot gas condult 42 or the stack 50. The detector 66 senses the concentratlon and thence the quantlty of NOx ln the flue gas stream after lt has left the second treatment zone 60.
Instrumentatlon for use as the detectors 62, 64, and 66 18 avallable commerclall~. The NO~
detectors 62 and 66, as well as the ammonla detector 64, are preferably the Model 6000 emlsslon monitoring sy~tem made b~ Alr Instrument Measurements, Inc.
Control of the NOx reductlon ln the flue gas ls achieved by uslng the results measured by the detectors 62, 64, and 66 to control the flows of treatment agents from the sources 51 and 56.
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Addltlonally, lnformation on the amount of fuel consumed, termed the boller load, and the combustlon temperature, -are provlded. The analysls of the results of the detectors and the control of the sources ls accompllshed by a controller 68. The controller 68 preferabl~ lncludes a mlcrocomputer programmed to follow the control procedures dlscussed next.
Flgure 5 deplcts the process for removlng NOx from the flow of flue gas. The flue gas 18 passed through the flrst treatment zone 58, numeral 70.
The treatment agent, preferably ures, 18 lntroduced into the flrst treatment zone 58 to ml~ with the flue gas, numeral 72. The NOx ln the flue gas then undergoes SNCR by reactlon wlth the treatment agent. The flue gas, wlth a reduced NOx content, passes through the second treatment zone 60 to undergo SCR, numeral 74. Optlonally, a second treatment agent, preferably ammonla, 18 a~ded at the upstream end of the second treatment zone 60, numeral 74. The NOx content of the flue gas 18 detected by detector 62 between the flrst trestment zone 58 and the second treatment zone 60, numeral 76. The ammonla sllp of the flue gas ls detected by detector 64 after the flue gas leaves the second treatment zone 60, numeral 78. In thls preferred embodlment, the NOx content of the flue gas 18 detected by detector 66 after the flue gas leaves the second treatment zone 60, numeral 80.
The measured values of the detectors 62, 64, and 66 are provlded to the controller 68, and the changes ln the flow rates of the treatment agents are determlned, numeral 80. The detalls of the procedure are descrlbed subsiequently ln relatlon to Flgures 2-4, but ln genera~, the procedures are as follows. The quantlty of urea added to the flrst treatment zone 58 ls lncreased, so long as the . .
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~i~4320 followlng crlterla are met: Flrst, the quantlty of NO~ measured by detector 62, over a perlod of tlme, is generally decreaslng wlth lncreaslng addltlons of the treatment a8ent (l.e., a negatlve slope of the NOx-urea curve of Flgure 2 or Flgure ~), and, second, the quantlty of ammonla measured at the detector 64, over a perlod of tlme, ls generally below a predetermlned maxlmum llmlt. On the other hand, lf the quantlty of NOx measured b~ detector 62, over a perlod of tlme, 18 generally lncreaslng wlth lncreaslng addltlons of the flrst treatment agent (l.e., a posltlve slope ln the NOx-urea curve of Flgure 3), the urea additlon rate 18 reduced.
Ammonla 18 added to the flue gas from source 56, 80 long as the followlng crlterla are belng met: Flrst, the quantlty of NOx measured at detector 62, over a perlod of tlme, 18 not generall~
decreaslng wlth lncreaslng addltlons of the treatment agent, and, second, the quantlty of ammonia measured at detector 64, over a perlod of tlme, 18 generally below a predetermlned maxlmum llmlt. The addltlon of the second treatment agent 18 lncreased as long as the quantlty of NOx measured at detector 66, over a perlod of tlme, ha~ not substantlall~ reached (l.e., 18 greater than) a predetermlned target control polnt. After these determlnatlons have been made, the flrst treatment agent flow rate 18 responslvel~ varled by controlllng the source of the treatment agent 51, numeral 84, and the ammonla flow rate (lf any) 18 responslvely varled bg controlllng the ammonla source 56, numeral 86.
Flgure 6 deplcts one procedure that may be used b~ the controller 68 ln performlng the determlnatlons lndlcated at -numeral 82, and the relatlon of these determlnatlons to the determlnatlons of ga~ content 62, 64, and 66, and .
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the varglng of the treatment agent, here urea 84, and ammonla 86. In general, thls procedure provldes that the treatment agent 1s added to the maxlmum extent posslble, and changes ln the addltlon of the treatment agent are the prlmar~ controlled quantlty. The sgstem performance 18 then trlmmed with addltlons of the second treatment agent, lf needed and permltted bg the clrcumstances.
In thls proce~s, the boller load 1~ checked, numeral 100, and the combustlon temperature ls checked, numeral 102. If elther 18 not stable, l.e., lf elther has changed slnce the last check, a FLAG ls set to 1, numeral 104, to lndlcate that the selectlve noncatalgtlc reductlon process must flrst be optlmlzed. Otherwlse, the FLAG remalns as lt was. In each case, readlngs are then taken from the detector 62 and the detector 64. The ammonla sllp of the stack gas as measured by detector 64 ls compared wlth a preestabllshed regulatory llmlt, numeral 106. If the ammonla sllp 18 below the llmlt, the stack NOY content 18 measured, numeral 66. If the ammonla sllp 18 not below the llmlt, a varlable STEP whlch lndlcatea the next change ln a flow rate 18 set to a negatlve value, numeral 108.
If the path through the NOX measurement, numeral 66 18 followed, the stack NOx content 18 compared wlth a preestabllshed goal, numeral 110. If the stack NOx content as measured bg detector 66 18 less than the goal, the process proceeds to the settlng of STEP to a negatlve value, numeral 108.
All paths pass through a check of the FLAG, numeral 114. If FLAG ls 1, a preestabllshed waltlng tlme after a change ln the flow rate of the treatment agent ls checked, numeral 116, and the change ln the NOx content at detector 62 after a wait time is checked, numeral 118. The slope of the curve of NOx as a functlon of flow rate of the flrst ;:
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treatment agent 18 thereby evaluated. If the slope is not between two preselected control polnts, the direction of the variation in the urea flow rate 51 (the prevlously established value of STEP) is selected, numeral 122, and the urea flow rate is varied, numeral 84. In the case where the slope ls less than the lower preselected control polnt, STEP
is set positive. If the slope 1~ hlgher than the upper preselected control polnt, STEP ls set negatlve. If the slope is between the two preselected control points, FLAG 18 set equsl to zero, numeral 124, and a change ln the ammonla flow rate 56 ls lnltlated, numeral 126. After waltlng a preselected amount of tlme, numeral 128, the ammonla flow rate change 18 lmplemented, numeral 86.
If FLAG 18 found to be zero at numeral 114, the process sklps dlrectly to numeral 128.
Thls entlre control procedure then repeats contlnuously, controlllng the urea and ammonla lnJectlon rates 80 that the N0~ snd ammonla are wlthln establlshed llmlts.
The approach of the lnventlon provldes a process for contiolllng the reductlon of NOx ln flue gas, whlle avoldlng ammonla sllp above establlshed llmlts. It 18 usable on a varlety of power plants and other combustlon devlces. Although a partlcular embodlment of the lnventlon has been descrlbed ln detall for purposes of lllustratlon, varlous modlflcatlons ma~ be made wlthout dapartlng from the splrlt and scope of the lnventlon. Accordlngl~, the lnventlon 18 not to be llmlted except as by the appended clslms.
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Claims (27)
1. A method of reducing NOx from a flow of flue gas produced by a burner, comprising the steps of:
passing the flue gas through a first treatment zone;
substantially simultaneously with said passing, selectively introducing a nitrogeneous treatment agent into the first treatment zone for the selective non-catalytic reduction of a first portion of the NOx from the flue gas passing through the first treatment zone, said introducing including an excessive amount of such nitrogeneous treatment agent above that required for such non-catalytic reduction of the first portion;
passing the flue gas, containing such excessive amount of the nitrogeneous treatment agent, through a second treatment zone which includes a catalyst therein, wherein a second portion of the NOx within the flue gas is removed therefrom as a result of a reaction thereof with such nitrogeneous treatment agent or its derivatives in the presence of the catalyst;
detecting the quantity of NOx in the flue gas intermediate the first and second treatment zones;
detecting the quantity of ammonia in the flue gas exiting from the second treatment zone;
increasing the quantity of nitrogeneous treatment agent introduced into the first treatment zone in response to said first and second mentioned detecting, so long as the following criteria are being met, A. the quantity of NOx indicated at said first mentioned detecting is generally decreasing with an increasing addition of the nitrogeneous treatment agent, and B; the quantity of ammonia detected at said second mentioned detecting is generally below a predetermined maximum limit.
passing the flue gas through a first treatment zone;
substantially simultaneously with said passing, selectively introducing a nitrogeneous treatment agent into the first treatment zone for the selective non-catalytic reduction of a first portion of the NOx from the flue gas passing through the first treatment zone, said introducing including an excessive amount of such nitrogeneous treatment agent above that required for such non-catalytic reduction of the first portion;
passing the flue gas, containing such excessive amount of the nitrogeneous treatment agent, through a second treatment zone which includes a catalyst therein, wherein a second portion of the NOx within the flue gas is removed therefrom as a result of a reaction thereof with such nitrogeneous treatment agent or its derivatives in the presence of the catalyst;
detecting the quantity of NOx in the flue gas intermediate the first and second treatment zones;
detecting the quantity of ammonia in the flue gas exiting from the second treatment zone;
increasing the quantity of nitrogeneous treatment agent introduced into the first treatment zone in response to said first and second mentioned detecting, so long as the following criteria are being met, A. the quantity of NOx indicated at said first mentioned detecting is generally decreasing with an increasing addition of the nitrogeneous treatment agent, and B; the quantity of ammonia detected at said second mentioned detecting is generally below a predetermined maximum limit.
2. A method as specified in claim 1, wherein the nitrogeneous treatment agent is a source of ammonia.
3. A method as specified in claim 1, wherein the nitrogeneous treatment agent is urea.
4. A method as specified in claim 1, wherein the steps of detecting are conducted substantially continuously.
5. A method as specified in claim 1, including the additional step of detecting the quantity of NOx in the flue gas exiting from the second treatment zone, said continuously increasing is in response to said last mentioned detecting, in addition to said first and second mentioned detecting, and said criteria to be met also include, C. the quantity of NOx indicated at said last mentioned detecting has not substantially reached a predetermined target point.
6. A method as specified in claim 1, including the additional step of introducing a second nitrogeneous treatment agent into the flue gas, said last mentioned introducing being downstream of the first treatment zone, and within or upstream of the second treatment zone.
7. A method as specified in claim 6, wherein the second nitrogeneous treatment agent is a source of ammonia.
8. A method as specified in claim 6, wherein the second nitrogeneous treatment agent is ammonia.
9. A method of reducing NOx from a flow of flue gas produced by a burner, comprising the steps of:
passing the flue gas through a first treatment zone;
substantially simultaneously with said passing, selectively introducing a nitrogeneous treatment agent into the first treatment zone for the selective non-catalytic reduction of a first portion of the NOx from the flue gas passing through the first treatment zone, said introducing including an excessive amount of such nitrogeneous treatment agent above that required for such non-catalytic reduction of the first portion;
passing the flue gas, containing such excessive amount of nitrogeneous treatment agent, through a second treatment zone which includes a catalyst therein, wherein a second portion of the NOx within the flue gas is removed therefrom as a result of a reaction thereof with such excessive nitrogeneous treatment agent in the presence of the catalyst;
detecting the quantity of NOx in the flue gas intermediate the first and second treatment zones;
detecting the quantity of ammonia in the flue gas exiting from the second treatment zone;
varying the treatment of the flue gas according to the steps of (1) increasing the quantity of nitrogeneous treatment agent introduced into the first treatment zone in response to said first and second mentioned detecting, so long as the following criteria are being met, A1. the quantity of NOx indicated at said first mentioned detecting is generally decreasing with an increasing addition of the nitrogeneous treatment agent, and B1. the quantity of ammonia detected at said second mentioned detecting is generally below a predetermined maximum limit, (ii) selectively introducing a second nitrogeneous treatment agent into the flue gas, said last mentioned introducing being downstream of the first treatment zone and within the second treatment zone, so long as the following criteria are being met, A2. the quantity of NOx indicated at said first mentioned detecting is not generally decreasing with an increasing addition of the nitrogeneous treatment agent, and B2. the quantity of ammonia detected at said second mentioned detecting is generally below a predetermined maximum limit.
passing the flue gas through a first treatment zone;
substantially simultaneously with said passing, selectively introducing a nitrogeneous treatment agent into the first treatment zone for the selective non-catalytic reduction of a first portion of the NOx from the flue gas passing through the first treatment zone, said introducing including an excessive amount of such nitrogeneous treatment agent above that required for such non-catalytic reduction of the first portion;
passing the flue gas, containing such excessive amount of nitrogeneous treatment agent, through a second treatment zone which includes a catalyst therein, wherein a second portion of the NOx within the flue gas is removed therefrom as a result of a reaction thereof with such excessive nitrogeneous treatment agent in the presence of the catalyst;
detecting the quantity of NOx in the flue gas intermediate the first and second treatment zones;
detecting the quantity of ammonia in the flue gas exiting from the second treatment zone;
varying the treatment of the flue gas according to the steps of (1) increasing the quantity of nitrogeneous treatment agent introduced into the first treatment zone in response to said first and second mentioned detecting, so long as the following criteria are being met, A1. the quantity of NOx indicated at said first mentioned detecting is generally decreasing with an increasing addition of the nitrogeneous treatment agent, and B1. the quantity of ammonia detected at said second mentioned detecting is generally below a predetermined maximum limit, (ii) selectively introducing a second nitrogeneous treatment agent into the flue gas, said last mentioned introducing being downstream of the first treatment zone and within the second treatment zone, so long as the following criteria are being met, A2. the quantity of NOx indicated at said first mentioned detecting is not generally decreasing with an increasing addition of the nitrogeneous treatment agent, and B2. the quantity of ammonia detected at said second mentioned detecting is generally below a predetermined maximum limit.
10. A method as specified in claim 9, including the additional step of detecting the quantity of NOx in the flue gas exiting from the second treatment zone, said continuously increasing is in response to said last mentioned detecting, in addition to said first and second mentioned detecting, and said criteria to be met also include, C1. the quantity of NOx indicated at said last mentioned detecting has not substantially reached a predetermined target point.
11. A method as specified in claim 9, including the additional step of detecting the quantity of NOx in the flue gas exiting from the second treatment zone and, in response to such last mentioned detecting in addition to said first and second mentioned detecting, increasing the amount of second nitrogeneous treatment agent introduced so long as said criteria to be met further includes C2. the quantity of NOx detected at said last mentioned detecting has not substantially reached a predetermined target control point.
12. A method as specified in claim 9, wherein said second treatment zone includes at least two subzones therein, the first such subzone including a fixed catalyst therein, and the second subzone including a heat exchanger having at least some catalyst-coated heat transfer elements disposed within the flow path of the flue gas stream.
13. A method as specified in claim 12, wherein said first subzone is upstream of the second subzone.
14. A method as specified in claim 12 wherein said last mentioned introducing is upstream of the second subzone.
15. A method as specified in claim 12 wherein the second nitrogeneous treatment agent is introduced at a location substantially adjacent to the entry end of the flue gas stream into the first subzone.
16. A method as specified in claim 12, wherein the heat exchanger is a recuperative heat exchanger.
17. A method as specified in claim 12, wherein the heat exchanger is a regenerative heat exchanger.
18. A method as specified in claim 12, wherein the heat exchanger is a rotary wheel heat exchanger.
19. A method as specified in claim 10 wherein said second treatment zone includes at least two subzones therein, the first such subzone including a fixed catalyst therein and the second subzone includes catalyst-coated heat transfer elements carried on the fixed internal hub of a regenerative air heater which is in communication with a duct assembly which directs the flue gas therethrough.
20. A method as specified in claim 9, wherein the temperature of the flue gas in the first.
treatment zone is from about 1600F to about 2000F.
treatment zone is from about 1600F to about 2000F.
21. A method as specified in claim 9, wherein the temperature of the flue gas in the second treatment zone is from about 300F to about 950F.
22. A method of reducing NOx from a flow of flue gas produced by a burner, comprising the steps of:
passing the flue gas through a first treatment zone:
substantially simultaneously with said passing, selectively introducing a nitrogeneous treatment agent into the first treatment zone for the selective non-catalytic reduction of a first portion of the NOx from the flue gas passing through the first treatment zone, said introducing including an excessive amount of such nitrogeneous treatment agent above that required for such non-catalytic reduction of the first portion;
passing the flue gas, containing such excessive amount of the nitrogeneous treatment agent, through a second treatment zone which includes a catalyst therein, wherein a second portion of the NOx within the flue gas is removed therefrom as a result of a reaction thereof with such nitrogeneous treatment agent or its derivatives in the presence of the catalyst;
detecting the quantity of NOx in the flue gas intermediate the first and second treatment zones;
detecting the quantity of ammonia in the flue gas exiting from the second treatment zone;
varying the quantity of nitrogeneous treatment agent introduced into the first treatment zone in response to said first and second mentioned detecting, by increasing the quantity of nitrogeneous treatment agent added so long as the following criteria are being met, A1. the slope of NOx with increasing addition of the treatment agent at said first mentioned detecting is generally negative, and B1. the quantity of ammonia detected at said second mentioned detecting is generally below a predetermined maximum limit, and by decreasing the quantity of nitrogeneous treatment agent so long as the following criterion is met, C1. the slope of NOx with increasing addition of the treatment agent at said first mentioned detecting is generally positive; and selectively introducing a second nitrogeneous treatment agent into the flue gas, said last mentioned introducing being downstream of the first treatment zone and no later than the entry of the flue gas into the second treatment zone, so long as the following criteria are being met, A2. the slope of NOx with increasing addition of the treatment agent at said first mentioned detecting is not generally negative, and B2. the quantity of ammonia detected at said second mentioned detecting is generally below a predetermined maximum limit.
passing the flue gas through a first treatment zone:
substantially simultaneously with said passing, selectively introducing a nitrogeneous treatment agent into the first treatment zone for the selective non-catalytic reduction of a first portion of the NOx from the flue gas passing through the first treatment zone, said introducing including an excessive amount of such nitrogeneous treatment agent above that required for such non-catalytic reduction of the first portion;
passing the flue gas, containing such excessive amount of the nitrogeneous treatment agent, through a second treatment zone which includes a catalyst therein, wherein a second portion of the NOx within the flue gas is removed therefrom as a result of a reaction thereof with such nitrogeneous treatment agent or its derivatives in the presence of the catalyst;
detecting the quantity of NOx in the flue gas intermediate the first and second treatment zones;
detecting the quantity of ammonia in the flue gas exiting from the second treatment zone;
varying the quantity of nitrogeneous treatment agent introduced into the first treatment zone in response to said first and second mentioned detecting, by increasing the quantity of nitrogeneous treatment agent added so long as the following criteria are being met, A1. the slope of NOx with increasing addition of the treatment agent at said first mentioned detecting is generally negative, and B1. the quantity of ammonia detected at said second mentioned detecting is generally below a predetermined maximum limit, and by decreasing the quantity of nitrogeneous treatment agent so long as the following criterion is met, C1. the slope of NOx with increasing addition of the treatment agent at said first mentioned detecting is generally positive; and selectively introducing a second nitrogeneous treatment agent into the flue gas, said last mentioned introducing being downstream of the first treatment zone and no later than the entry of the flue gas into the second treatment zone, so long as the following criteria are being met, A2. the slope of NOx with increasing addition of the treatment agent at said first mentioned detecting is not generally negative, and B2. the quantity of ammonia detected at said second mentioned detecting is generally below a predetermined maximum limit.
23. A method as specified in claim 22, including the additional step of detecting the quantity of NOx in the flue gas exiting from the second treatment zone, said continuously increasing is in response to said last mentioned detecting, in addition to said first and second mentioned detecting, and said criteria to be met also include, D1. the quantity of NOx indicated at said last mentioned detecting has not substantially reached a predetermined target point.
24. A method as specified in claim 22, including the additional step of detecting the quantity of NOx in the flue gas exiting from the second treatment zone and, in response to such last mentioned detecting in addition to said first and second mentioned detecting, increasing the amount of second nitrogeneous treatment agent introduced so long as said criteria to be met further includes C2. the quantity of NOx detected at said last mentioned detecting has not substantially reached a predetermined target control point.
25. A method of reducing NOx from a flow of flue gas produced by a burner, comprising the steps of:
passing the flue gas through a first treatment zone and thereafter through a second treatment zone which includes a catalyst for the selective catalytic reduction of NOx in the flue gas;
controllably introducing a flow of a nitrogeneous treatment agent into the first treatment zone for the selective non-catalytic reduction of the NOx in the flue gas passing through the first treatment zone, the nitrogeneous treatment agent being introduced at a flow rate so as to minimize the quantity of NOx in the flue gas intermediate the first and second treatment zones, subject to the constraint that the quantity of ammonia in the flue gas exiting from the second treatment zone remain below a preselected value; and controllably introducing a flow of a second nitrogeneous treatment agent into the flue gas at a location between the two treatment zones, the second nitrogeneous treatment agent being introduced at a flow rate so as to further decrease the quantity of NOx in the flue gas exiting from the second treatment zone, subject to the constraint that the quantity of ammonia in the flue gas exiting from the second treatment zone remain below the preselected value.
passing the flue gas through a first treatment zone and thereafter through a second treatment zone which includes a catalyst for the selective catalytic reduction of NOx in the flue gas;
controllably introducing a flow of a nitrogeneous treatment agent into the first treatment zone for the selective non-catalytic reduction of the NOx in the flue gas passing through the first treatment zone, the nitrogeneous treatment agent being introduced at a flow rate so as to minimize the quantity of NOx in the flue gas intermediate the first and second treatment zones, subject to the constraint that the quantity of ammonia in the flue gas exiting from the second treatment zone remain below a preselected value; and controllably introducing a flow of a second nitrogeneous treatment agent into the flue gas at a location between the two treatment zones, the second nitrogeneous treatment agent being introduced at a flow rate so as to further decrease the quantity of NOx in the flue gas exiting from the second treatment zone, subject to the constraint that the quantity of ammonia in the flue gas exiting from the second treatment zone remain below the preselected value.
26. A method as specified in claim 25, wherein the steps of controllably introducing a flow of a nitrogeneous treatment agent and controllably introducing a flow of a second nitrogeneous treatment agent are implemented incrementally, so that the flow rate of the nitrogeneous treatment agent is determined prior to the determination of the flow rate of the second nitrogeneous treatment agent.
27. A method as specified in claim 25, including the additional step, after the step of controllably introducing a flow of a second nitrogeneous treatment agent, of incrementally repeating the steps of controllably introducing a flow of a nitrogeneous treatment agent and controllably introducing a flow of a second nitrogeneous treatment agent.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US07/933,488 US5233934A (en) | 1992-08-20 | 1992-08-20 | Control of NOx reduction in flue gas flows |
| US933,488 | 1992-08-20 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2104320A1 true CA2104320A1 (en) | 1994-02-21 |
Family
ID=25464061
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002104320A Abandoned CA2104320A1 (en) | 1992-08-20 | 1993-08-18 | Control of nox reduction in flue gas flows |
Country Status (8)
| Country | Link |
|---|---|
| US (1) | US5233934A (en) |
| EP (1) | EP0583771B1 (en) |
| KR (1) | KR970001439B1 (en) |
| AT (1) | ATE162103T1 (en) |
| AU (1) | AU655115B2 (en) |
| CA (1) | CA2104320A1 (en) |
| DE (1) | DE69316286D1 (en) |
| ES (1) | ES2114983T3 (en) |
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- 1992-08-20 US US07/933,488 patent/US5233934A/en not_active Expired - Fee Related
-
1993
- 1993-08-17 ES ES93113131T patent/ES2114983T3/en not_active Expired - Lifetime
- 1993-08-17 AT AT93113131T patent/ATE162103T1/en not_active IP Right Cessation
- 1993-08-17 EP EP93113131A patent/EP0583771B1/en not_active Expired - Lifetime
- 1993-08-17 DE DE69316286T patent/DE69316286D1/en not_active Expired - Lifetime
- 1993-08-18 CA CA002104320A patent/CA2104320A1/en not_active Abandoned
- 1993-08-19 AU AU44757/93A patent/AU655115B2/en not_active Ceased
- 1993-08-20 KR KR1019930016168A patent/KR970001439B1/en not_active IP Right Cessation
Also Published As
| Publication number | Publication date |
|---|---|
| KR940003603A (en) | 1994-03-12 |
| ATE162103T1 (en) | 1998-01-15 |
| AU4475793A (en) | 1994-03-03 |
| KR970001439B1 (en) | 1997-02-06 |
| US5233934A (en) | 1993-08-10 |
| EP0583771A1 (en) | 1994-02-23 |
| AU655115B2 (en) | 1994-12-01 |
| DE69316286D1 (en) | 1998-02-19 |
| ES2114983T3 (en) | 1998-06-16 |
| EP0583771B1 (en) | 1998-01-14 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| FZDE | Discontinued |